This invention relates to apparatus for correcting irregularities in a textile strand utilizing a capacitance related transducer wherein the effects of moisture content are compensated for.
Drawing frames perform a doubling process, which drafts or attenuates and reduces a number of incoming textile slivers, typically eight, to a single output or delivery sliver. This may be considered to be an averaging function by which the magnitude and length of thick/thin variations in the incoming sliver are reduced in the outgoing sliver.
In addition, drawing frames frequently include a levelling function. In levelling, the lineal mass of the incoming or the outgoing sliver, or both, is measured by an appropriate transducer. The output signal voltage is used to control a motor which corrects the drawing frame's draft ratio. This in turn regulates the uniformity of the output sliver.
The uniformity of the drawing frame's output lineal density is critical to the efficiency of down stream mill processes, and to the quality of the dependent textile yarn or fabric products.
Both the control of the sliver weight, and the measurement of its uniformity are dependent on suitable transducers or sensors. The sensing methods commonly used for these purposes have disadvantages in both accuracy and consistency. Volumetric methods include the tongue and groove roller or displacement method, and the air trumpet or pneumatic method. These methods rely on the measurement of physical or mechanical attributes of the fiber which while related to the lineal density, are subject to error.
The tongue and groove method is subject to variations in the resilience or compressibility of the fiber, and to the form factor or shape of the sample under compression. This method also disrupts the desirable laminar flow of fiber into the drafting zone when used as the in-feed sensor.
The air trumpet method uses the air entrapped in the sliver as a measure of its lineal density. When the sliver is passed through a restricted orifice, an air pressure is generated which is related to the density. However, the pressure is also dependent on the fiber velocity and on the fineness or micronaire of the fiber. The orifice may be fouled by oils and other materials borne by the fiber, particularly in the case of cotton. Other methods which have been proposed in this category are subject to similar disadvantages. These include measurement of the force required to pull the delivery sliver through the trumpet, and measurement of the audio-frequency noise, or the heat generated due to friction in the same area.
Another class of sensing which has been tried is the simple capacitive method as heretofore used. This method depends on the dielectric characteristic of the fiber. As fiber passes through the sensing zone, it replaces the air forming the dielectric of a capacitor, causing an increase in the effective dielectric constant, which is dependent on the amount of fiber present. This in turn causes a change in the capacitance value, which is detectable by a variety of electronic means, in order to produce an output voltage related to the lineal density of the sliver. However, many fibers, particularly cotton contain moisture, the dielectric constant of which is eight to ten times that of the fiber. Small changes in the moisture content thus cause major changes in the apparent dielectric constant, and hence in fiber weight measurements. This method is also subject to ambient temperature changes. Capacitive sensing has, however, been successfully applied in reading the rate of change of the lineal density as opposed to its absolute level.
U.S. Pat. Nos. 3,155,898, 3,155,900, 3,155,901, 3,155,902, 3,241,062, 3,290,588, 4,208,625 and 4,568,875 disclose detectors utilizing capacitance measurements.
Another class of sensor used in industry for true mass sensing is the beta gauge. This method uses a radiation source. However, the cost factors do not justify its use for textiles in its present form.